The prior art includes a number of universal joint gyroscope flexure hinges such as exemplified by U.S. Pat. Nos. 3,527,062; 3,538,776; 3,354,726; and 3,709,045, all of which are assigned to the present assignee.
The usual universal flexure hinge of the prior art, and as described in the aforesaid patents, comprises an inner hinge unit and an outer hinge unit, the hinge units being interconnected in a general concentric and parallel manner. Each of the two hinge units of the prior art assemblies is made up of three gimbals, together with two quadrature pairs of appropriately disposed flexure bars which interconnect the three gimbals, and which permit selective tilting of the gimbals about any transverse axis perpendicular to the central axis of the assembly.
The upper gimbals of the two hinge units of the prior art flexure hinge assembly are affixed to one another and to the flywheel of the gyroscope, and the lower gimbals of the two hinge units are connected to one another and to the spin shaft of the gyroscope. The inner hinge unit is constructed to provide axial stiffness along the spin axis of the gyroscope, and the outer hinge unit is constructed to provide the necessary radial and torsional stiffness to the assembly.
A disadvantage inherent in the above-described prior art flexure hinge assemblies is the inability to compensate the structure to eliminate rectified torques resulting from angular vibration inputs about its input axis. As a result, a rectified drift-producing torque will occur when angular vibration inputs about the input axis occur at twice gyroscopic flywheel rotation frequency.
The improvement of U.S. Pat. No. 3,709,045 relates to the construction of a universal joint assembly so that it incorporates compensation weights on a middle inner gimbal and on a middle outer gimbal, so that these gimbals can be adjusted for inertia and mass unbalance to compensate for spring rate torques and effective gimbal unbalance, and to compensate for the angular vibration rectified drift-producing torque. Although the hinge assembly of this patent produces generally satisfactory results, it is burdened with the disadvantage that there is a change in restraint as a result of spin axis acceleration. A wheel tilt combined with spin axis acceleration results in a torque and gyroscope drift. For high performance systems, this condition is not tolerable and must be compensated for at the system level, creating additional cost and potential reliability problems.
The restraint change is due to the inner hinge flexures. When a flexure is loaded in compression, its restraint will decrease and when it is loaded in tension its restraint will increase. For a flat simple flexure the change in restraint is approximately defined by EQU e=(PL/3)
where
e=change in restraint PA1 L=length of flexure PA1 P=axial load.
In the hinge assembly of the latter-mentioned U.S. patent, all the flexures for the inner hinge are either loaded in compression or tension, depending on load direction. As a result, the full change in restraint is experienced. Further, with the hinge assembly of the last-mentioned patent, it is difficult to trim the axial compliance to make it equal the radial hinge compliance. This equality is a desirable condition to minimize the rectified torques due to linear vibrations. The ability to trim this parameter on the assembled gyroscope wheel of the prior art patent was limited because there was no hinge section which could be readily trimmed to change the compliance without contaminating or damaging the wheel and hinge assembly.